Synchronization is a practical problem that all communications systems need to resolve. Without precise synchronization, reliable data transmission is impossible. Therefore, a synchronization technology is directly related to performance of an entire communications system.
Generally, synchronization is classified into timing synchronization and frequency synchronization. Timing synchronization includes coarse synchronization and fine synchronization, and a purpose is to enable a signal receive end to determine the start time and end time of each symbol. To enable the receive end to accurately find a beginning part of a data symbol sent by a transmit end, a current technical means that is commonly used is to add a preamble sequence (that is, a Preamble, where a symbol included in the Preamble may be referred to as a preamble symbol) before the data symbol to indicate arrival of the data symbol. Because the preamble sequence is known to the receive end, the receive end may perform a correlation calculation between the preamble sequence and a received signal sequence by using the known preamble sequence, to obtain a correlation value that represents a correlation degree between the preamble sequence and the received signal sequence. Specifically, when the receive end receives the preamble sequence, an apparent peak appears in the correlation value obtained by calculation according to the foregoing manner; therefore the receive end may determine that a currently received signal sequence is the preamble sequence and then perform timing synchronization.
Currently, a timing synchronization solution used in a narrow band-power line communication (NB-PLC) system is similar to the foregoing solution, and is also implemented based on the preamble sequence. For example, according to a provision, on a physical frame structure, specified by the International Telecommunications Union (ITU) in the G.hnem physical layer protocol G.9955, timing synchronization may be performed for a signal at the receive end by using a preamble sequence shown in FIG. 1.
The preamble sequence shown in FIG. 1 mainly includes preamble symbol S1 and preamble symbol S2, where S1 consists of eight orthogonal frequency division multiplexing (OFDM) symbols that carry same information. If S is used to represent a single OFDM symbol included in S1, S1 may be represented as: S1={S, S, S, S, S, S, S, S}; S2 includes only one OFDM symbol that is denoted as −S, that is, S2={−S}. In FIG. 1, a main purpose of using multiple consecutive OFDM symbols to repeatedly carry same information is to resist a bad channel condition in the NB-PLC system, so as to improve a probability of correctly receiving a preamble symbol by the receive end, thereby improving a synchronization rate of the NB-PLC system.
In practical application, when a receive end needs to determine a beginning part of a data symbol sent by a transmit end, delay autocorrelation may first be performed on a received signal sequence, that is, a delay correlation value of a pair of OFDM symbols that are consecutively received is calculated, where a length of the pair of OFDM symbols is a length of the signal sequence, and when a quantity of delay correlation values determined within a time period are more than a set quantity of correlation values, it is determined that a preamble symbol arrives, that is, a coarse synchronization process is completed. Further, the receive end performs sliding correlation between a locally prestored symbol S and signal sequences received after the coarse synchronization process is completed; and when a negative peak appears in correlation values, obtained by calculation, between the locally prestored symbol S and the received signal sequences, it is determined that S2 is detected, and therefore a timing synchronization point may be determined, that is, a fine synchronization process is completed.
Currently, the NB-PLC system mainly works in a frequency band of 3-500 kHz, and a channel of the NB-PLC system mainly includes a slowly-changing multipath, colored noise, narrowband interference, pulse interference, periodic noise whose frequency is synchronous with an industrial frequency, and periodic noise whose frequency is asynchronous with the industrial frequency. The periodic noise whose frequency is synchronous with the industrial frequency and the periodic noise whose frequency is asynchronous with the industrial frequency may be classified into frequency-domain narrowband interference and time-domain burst pulse interference. In a low band channel of the NB-PLC system, the frequency-domain narrowband interference is extremely severe. In addition, time-domain burst pulse interference that is generated because of non-standard electrical equipment in a power line and whose intensity is 10-15 dB higher than background noise also makes a channel condition of the NB-PLC system extremely bad.
Existence of the foregoing interference makes the preamble sequence shown in FIG. 1 vulnerable to damages. For example, S2 in FIG. 1 is extremely easy to be flooded by the burst pulse interference, thereby making it difficult to complete timing synchronization.